1. Field of the Invention
The present invention relates to a rear-projection-type display apparatus that projects an image formed by a small-sized display element on a large screen.
2. Description of the Related Art
Typically, a rear-projection-type display apparatus includes a light source, an optical component that guides a light beam emitted from the light source to a small-sized display element that forms an image, an optical engine that includes the small-sized display element and a projection lens that magnifies and projects the image, a large reflecting mirror that changes a direction of the light beam output from the projection lens, and a screen on which the light beam from the reflecting mirror is projected. These principal components are encased and held in place in a casing of the rear-projection-type display apparatus.
The dimensions of the casing of the rear-projection-type display apparatus are virtually determined by the contour of the optical engine and constraints on a beam divergence angle of the light beam output from the projection lens. Various rear-projection-type display apparatuses have been proposed for a low profile design under such constraints. For example, a rear-projection-type display apparatus disclosed in Japanese Patent Application Laid-open No. 2006-292901 includes a projection lens including a plurality of refractive lenses each having a rotationally symmetrical surface geometry and a set of two lenses each having at least one free-form rotationally asymmetrical surface, an optical system that includes at least one reflecting mirror having a free-form rotationally asymmetrical reflecting surface, and a plane reflecting mirror that reflects a light beam from the optical system to a screen.
However, in the rear-projection-type display apparatus disclosed in Japanese Patent Application Laid-open No. 2006-292901, the projection lens is constructed of two lens groups. Therefore, the entire optical system becomes bulky and complicated, and the manufacturing cost increases. Furthermore, a rotationally asymmetrical reflecting surface is difficult to produce thereby leading to cost increase.